US3707667A - Unity input displacement factor frequency changer - Google Patents

Abstract

The invention relates to a frequency changer (or cycloconverter) system employing conduction controlled bilateral switches operating in a controlled switching mode to maintain a unity input displacement factor for the frequency changer system independent of load power factor and output voltage. The input displacement factor, which is often referred to as the input power factor, of the frequency changer system is defined to be the angle between the input, or source, voltage and the fundamental component of the input current drawn by the frequency changer system.

Description

United States Patent Gyugyi 51 Dec. 26, 1972 [541 UNITY INPUT DISPLACEMENT FACTOR FREQUENCY CHANGER [72] Inventor: Laszlo Gyugyi, Penn Hills, Pa.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Nov. 12, 1971 [21] Appl. No.: 198,283

Related US. Application Data [63] fgiinuation-in-part of Ser. No. 84,796, Oct. 28,

[52] US. Cl. ..321/5, 321/7, 321/69 R [51] Int. Cl. ..H02m 5/22 [58] Field of Search ..321/5, 7, 69 R [56] References Cited UNITED STATES PATENTS 3,170,107 Jessec ..321/7 X 3,178,630 4/1965 Jessee ..321/7 3,419,785 12/1968 Lafuze ..321/5 X 3,431,483 3/1969 Lafuze ..321/7 3,470,447 9/1969 Gyugyi et a1. ..321/7 3,493,838 2/1970 Gyugyi et a1. ..321/7 Primary Examiner-William M. Shoop, Jr. Attorney-F. H. Henson, C. F. Renz and M. P. Lynch [57] ABSTRACT The invention relates to a frequency changer (or cycloconverter) system employing conduction controlled bilateral switches operating in a controlled switching mode to maintain a unity input displacement factor for the frequency changer system independent of load power factor and output voltage. The input displacement factor, which is often referred to as the input power factor, of the frequency changer system is defined to be the angle between the input, or source, voltage and the fundamental component of the input current drawn by the frequency changer system.

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l/2 Ol UNITY INPUT DISPLACEMENT FACTOR FREQUENCY CHANGER CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation in part of application Ser. No. 84,796, entitled Unity Input Displacement Factor Frequency Changer, filed Oct. 28, [970. This application is also related to a second cofiled continuation in part application Serial No. 198,2l5 of application Ser. No. 84,796.

BACKGROUND OF THE INVENTION In conventional direct AC to AC frequency changer systems the input displacement factor of such systems is dependent upon the power factor of the load at the output. The relationship between the input displace ment factor and the load power factor may be linear or non-linear depending upon the operating principle of a particular frequency changer. For example, the naturally commutated (or phase controlled) frequency changer always has a lagging input displacement factor independent of the load power factor. The input displacement factor of the naturally commutated frequency changer is thus non-linearly related to the load power factor such that a unity power factor load, or resistive load, is seen by the power source as an inductive load, and a purely capacitive load or a purely inductive load as seen by the power source as a pure inductance. In the case of the force commutated mode of operation of a frequency changer, as described in US. Pat. No. 3,493,833 entitled Static Frequency Converter with Novel Voltage Control, which is assigned to the assignee of the present invention, there is a direct linear relationship between the load power factor and the input displacement factor, i.e., the input displacement factor is equal to the load power factor.

In many applications it would be advantageous to have a frequency changer system with a load-independent, unity input displacement factor such that the input power source would be required to supply only the resistive component of the load current, i.e., the current portion which is actually consumed. In applications where the input power is produced by, a local generator this would result in a weight and size reduction for the generator and electrical cables, as well as providing improved overall efficiency. In other applications such as high power AC motor drives, a frequency changer system having a unity input displacement factor would reduce installation and operation expenses.

SUMMARY OF THE INVENTION The novel frequency changer systems comprising this invention is a unity input displacement frequency changer system which draws only the resistive component of the load current from the power source regardless of the power factor of the load. The unity input displacement factor characteristic of the frequency changer system is maintained for any load ranging from a pure capacitance to a pure inductance. In other words the rated inductive or capacitive load current can be supplied without actually drawing any fundamental current from the input power source.

The unity input displacement factor mode of operation of a pair of converters comprising a frequency changer system is described and illustrated with reference to two independent configurations; In a first configuration two converters employing bilateral switches being supplied from the same input power source are operated such that the one of the converters transfers back to the input terminals the actual load power factor while the other converter transfers back an inverted representation or mirror image of the actual load power factor. The combination of the actual and inverted load power factor at the input terminal results in a zero reactive load current flow through the input power source and thus establishes a unity input displacement factor for the frequency changer system.

In the second configuration each of the converters exhibits a unity displacement factor and the combination of the two converters results in a substantial reduction in input current and output voltage distortion while maintaining a unity input displacement factor. The output voltage waveforms of each of the converters are generated by appropriately phase modulating the firing angles of the bi-lateral switches in each converter with respect to the phases of the supply voltages. The modulation is controlled so as to produce complementing output wave shapes from the converters with each exhibiting the same mean output voltage. This is achieved by maintaining the sum of the firing angles of the two converters equal to It is noted that this method of basic control is essentially the same as that used for controlling the firing angles of a naturally commutated frequency changer. Thus, existing control techniques, such as the sine wave crossing method, or integral lining control method can be used to obtain the proper phase modulation. The similarity ends at this point, however, in that the operation of the frequency changer system comprising this invention is completely different from that of the naturally commutated frequency changer. The naturally commutated frequency changer consists of a positive conducting converter and a negative conducting converter such that each converter normally operates only during one-half cycle of the output current and remains idle during the other half cycle. The two converters of the naturally commutated frequency changer can be maintained in continuous conduction only if a circulating current is allowed to flow between them via the input supply source. In either of these modes of operation, however, the input displacement factor of the naturally commutated frequency changer is less than unity.

The. frequency changer system comprising the second configuration consists of two converters employing bilateral switches. Each of the converters is capable of conducting both positive and negative current. Thus both converters are in conduction during the total output cycle, each one supplying half of the load current. There is no significant circulating current between the converters and both converters exhibit unity displacement factors at their input.

In both configurations where two converters are employed the total rms distortion of the input current as well as the rms distortion of the output voltage is significantly reduced.

The invention will become more readily apparent from the following exemplary description in connection with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a basic schematic illustration of a frequency changer utilizing bi-lateral switching elements;

FIGS. 2A and 2B are waveform illustrations of two switching modes of operations for the bi-lateral switching elements of FIG. 1;

FIG. 3A is a schematic illustration of a frequency converter system utilizing two frequency converters connected in parallel by means of interphase reactor;

FIG. 3B is a waveform illustration of the operation of the embodiment of FIG. 3A;

FIG. 4A is a schematic illustration of an embodiment of the clock pulse generator of FIG. 3A;

FIG. 4B is a waveform illustration of the operation of the clock pulse generator of FIG. 4A;

FIG. 5A is a schematic illustration of a six pulse frequency changer system comprised of interphase reactor coupled frequency changers as illustrated in FIG. 3;

FIG. 5B is a block diagram of a control circuit for a six pulse frequency changer system of the type illustrated in FIG. 5A;

FIGS. 6A and 6B are waveform illustrations of the operation of a frequency changer system of FIGS. 5A and 53;

FIG. 7 is a graphical illustration of the operation of the six pulse frequency converter system of FIG. 5 according to the invention;

FIG. 8 is a schematic illustration of a three-phase output frequency changer system comprised of sixpulse bridge type frequency changers as illustrated in FIG. 5;

FIGS. 9A is a graphical illustration of the operation of the frequency changer system of FIG. 8 according to an alternate embodiment of the invention.

FIG. 10A is schematic illustration ofa control circuit of the type used in the embodiment of FIG. 8;

FIGS. 10B and 10C are waveform illustrations of the operation of the embodiments of FIGS. 8 and 10A; and

FIGS. 11A and Rare waveform illustrations of an alternate mode of operation of the embodiment of FIG. 10A.

DESCRIPTION OF THE PREFERRED EMBODIMENT The novel configurations for achieving unity input displacement factor for frequency changer systems independent of load power factor and, input and output frequency, will be described separately. FIGS. 1-8 correspond primarily to the discussion of one configuration, while FIGS. 8-11, which are further extensions of FIGS. l-8, correspond to the second configuration.

Referring to FIG. 1 there is illustrated schematically a frequency changer 10 comprised of generator G, bilateral switches 00, and electrical load L. The bilateral switches QQ are typically represented as a diode bridge b in combination with a transistor t. The ON- OFF control of the transistors provided by control circuit 18 in combination with the current conducting paths provided by the diode bridges enables the bilateral switches 00 to conduct current in both the forward and reverse directions. The current conduction to load L from the generator phases A, B and C provided by the controlled conduction of the bilateral switches O01. Q02 and 003, respectively, develops the individual phase voltages across the load L.

According to the basic mode of operation of such a frequency changer as described in US. Pats. No. 3,148,323 issued to Blake et al. Sept. 8, I964; 3,170,107 issued to Jessee Feb. 16, 1965; and 3,493,838 issued to Gyugyi et al. Feb. 3, I970 These US. Patents, the latter two being assigned to the assignee of the present invention, are incorporated herein by reference. Each of the bilateral switches 00 are allowed to conduct for a fixed period of time T, such that the input voltages corresponding to the input generator phases A, B and C are successively connected to the load L for the same time interval T, resulting in the fabrication of a predominantly sinusoidal output waveform across the load L. It is shown in the aboveidentified US. Pat. No. 3,148,323 that the fundamental component of the generated output voltage has a frequency j, (f/3) f, where f,, equals HT, 3 represents the number of bilateral switches and corresponds to the pulse number of the converter and f, is the supply frequency. This expression indicates that there are two possible values off,,, one greater and one less than 3f, which will result in the same output frequency. If f /3 is greater than f,, then the output frequency,f,, is given by:

fo fiu/ fl If f /3 is less than f,, then f takes the following form fo=f,(fp2/ (2) The output voltage waveshapes corresponding to the switching frequencies f, andf respectively are shown in FIGS. 2A and 2B. It can be shown that the fundamental components of the two waveforms are identical and correspond to the following expressions:

.xn=f., in 2wf.: 1 4) It can be shown that the phase relationship between an input phase voltage, i.e., V,, of the generator G and the dominant, or fundamental input current I drawn by the frequency changer 10 corresponding to the switching frequencies f,, and f can be described mathematically as follows: if one of the input phase voltages is expressed as:

A V,(t) V,sin(2'rrf,t) then the current I will be I =(3 V 3/277) I ,sin(21rf,tl- (6) when the frequency changer is operated by the switching frequency f,, (i.e., ]",,,/3 f,), and the input current will be I,, =(3 V 3/21r) 1 ,sin(21r1rf,t I (7) when the frequency changer is operated by switching frequency f (i.e., f,, /3 f,). It is observed from the above equations (6) and 7) that while the magnitude of the input current remains unchanged for both switching frequencies and f the phase angle of the input currents corresponding to the respective switching frequencies are opposite.

It can therefore be concluded that if a frequency changer operates atthe frequency f,,, it will result in an input displacement angle of the frequency changer which is the negative of the load phase angle, and if the frequency changer operates at the switching frequency f the resulting input displacement angle will be identical to the load phase angle. The amplitude of the input current at both switching frequencies remains the same.

The significance of the above relationship of the current and voltage of the frequency changer operating at the switching frequencies f,,, and f,, will become more apparent from the following discussion in reference to FIGS. 3A and 3B.

Referring to FIG. 3A there is illustrated a frequency changer system 20 comprised of supply voltage generator G, two converters 22 and 24, the outputs of which are connected in parallel via an interphase reactor 26 to the electrical load L. The ON-OFF conduction of the bilateral switches of converters 22 and 24 are controlled bia conduction control circuits 28 and 30 by clock pulse generator 32.

The operation of the frequency changer system 20 is illustrated by the waveforms l l l of FIG. 3B.

The clock pulse generator 32 provides two strings of pulses P, and P illustrated in waveforms (1) and (2) respectively, having frequencies off,,, and f respectively, as defined by equations l and (2). In order to ensure that the fundamental components of the two composite output waveforms V and V illustrated in waveforms (9) and (10) respectively are in phase, clock pulse wave P is shifted by one half of its period time, f with respect to clock pulse wave P, as illustrated in FIG. 3B. The phase relationship between the two coordinated clock pulse waves and the supply voltages V V and V is somewhat arbitrary; the one shown in FIG. 3B was chosen for the purpose of clear illustration. The actual generation of clock pulse waves P, and P having the required frequency and phase relationship will be explained later.

Clock pulses P, and P are fed to 3-stage ring counters 34 and 36 which develops the ON-OFF conduction control waves, I1, I2, I3 illustrated in waveforms (3), (4) and (5) respectively and I11, I12, II3 illustrated in waveforms (6), (7) and (8) respectively. These control waves are appropriately amplified by two three channel amplifiers, 38 and 40, and then coupled to the control electrodes of the bilateral switches of converters 22 and 24. In the waveform illustrations of FIG. 38 it is assumed that each bi-lateral switch becomes conductive whenever the corresponding control wave is positive, and it ceases conduction when the control wave is zero.

The above described sequential operation of bilateral switches QQl, QQ2 and QQ3 of converter 22 and 24 generate output wave shapes V and V from the input supply waves V V and V,- as illustrated by waveforms (9) and (10) in FIG. 38. Since in the above detailed mode of operation, the converter 22 is operated at a switching frequency f,,,/3 and converter 24 is operated at the switching frequency f,, /3 such that equations (1 (2) and (3) are simultaneously satisfied, ie both converters generate an output voltage wave shape with the same fundamental components, it follows that the frequency changer system 20 will generate an output voltage wave shape V,, illustrated in waveform (11) of FIG. 3B, which is the arithmetic means of the constituent waves V and V as illustrated in waveforms (9) and (10) of FIG. 3B. In this mode of operation, therefore, the fundamental component of output waveform V, is also defined by equation (3).

While the expression for the fundamental component of the voltage output waveform of the frequency changer system 20 remains the same as the expression for the fundamental component of the voltage output waveform of the frequency changer system of FIG. 1, the fundamental component of the input current for the frequency changer system 20 becomes where L is the amplitude of the real component of the load current and is defined according to the equation:

It is apparent from equation (8) that the operation of the converters 22 and 24 at the switching frequencies (f,,,/3) and ([1 3) respectively isolates the input power source G from the reactive component of the load current thereby establishing a unity input displacement factor for the frequency changer system 20 regardless ofload power factor and output frequency.

This novel operation of the combined frequency changers 22 and 24 of FIG. 3A provides an additional benefit in that the total rms distortion in the output voltage waveform is reduced as is apparent from a comparison between the component waveforms of FIG. 3B and the resultant waveform. It has been determined that this rms distortion is reduced by a factor of l VT As was explained above, in the described mode of operation of frequency changer 20 the two clock pulse waves P, and P must have the previously defined frequency and phase relationship. It is obvious that there are numerous ways of implementing the operation of clock pulse generator 32 to produce the series of pulses required to operate the control circuits 28 and 30 in the desired manner. A possible implementation of the clock pulse generator 32, utilizing the well known principles of the sine wave crossing control, is shown in FIG. 4A and its operation is illustrated by the waveforms (l)-(9) of FIG. 4B.

The clock pulse generator is comprised of transformers 322, 324, 326, producing three sinusoidal timing waves which are in antiphase with the supply voltages, a reference generator 328 generating a sinusoidal waveform of the wanted output frequency, 3-pulse sine wave crossing detector circuits 330 and 332, an integrator 334, null detector 336, a flip-flop circuit 338, logic AND gates 340, 342, 344 and 346 and logic OR gates 348 and 350.

The operation of the clock pulse circuit 32 may be explained as follows with reference to FIG. 4B.

The three timing waveforms, --V,,, V,, and V are compared to reference wave V In the present discussion it is assumed that the amplitude of the reference wave is approximately equal to, but not greater than IOEIOIZ 0098

Claims (5)

1. An AC to AC frequency changer apparatus for developing an AC output voltage across a load, comprising, in combination, an AC input power source, converter circuit means including a plurality of ON-OFF conduction controlled bi-lateral switching means operatively coupled between said AC input power source and said load, said converter circuit means comprised of a first and second converter circuit, a first conduction control circuit for controlling the conduction of said ON-OFF bilateral switching means associated with said first converter circuit to develop a first output voltage waveform and to produce an input phase angle for said first converter circuit of the same polarity as the load phase angle, second conduction control circuit means for controlling the conduction of said ON-OFF bilateral switching means associated with said second converter circuit to develop a second output voltage waveform and to produce an input phase angle for said second converter circuit of a polarity opposite to the polarity of said load phase angle, and means for combining said first and second output waveforms to form said AC output voltage across said load, the combination of said first and second converter circuits resulting in a net zero input phase angle for said converter circuit means, and consequently a unity input displacement factor for said converter circuit means.

2. An AC to AC frequency changer apparatus as claimed in claim 1 wherein said first conduction control circuIt means establishes a switching frequency for said ON-OFF conduction controlled bi-lateral switching means associated with said first converter circuit that is lower than the frequency of said AC input power source.

3. An AC to AC frequency changer apparatus as claimed in claim 1 wherein said second converter circuit means establishes a switching frequency for said ON-OFF conduction controlled bi-lateral switching means associated with said second converter circuit that is higher than the frequency of said AC input power source.

4. An AC to AC frequency changer apparatus for developing an AC output voltage across a load, comprising, in combination, an AC input power source, a converter circuit means including a plurality of ON-OFF conduction controlled bi-lateral switching means forming a first and second converter circuit, each converter circuit having a pulse number P, wherein P represents the number of bi-lateral switching means employed in each converter circuit, said first and second converter circuits being operatively connected between said AC input power source and said load, each of said bi-lateral switching means being capable of conducting current in both directions between said input power source and said load, first conduction control circuit means for generating a first conduction control signal to control the ON-OFF conduction controlled bi-lateral switching means associated with said first converter circuit, each bi-lateral switching means of said first converter circuit being rendered conductive by said conduction control signals for a period of time, T1, to develop a first output voltage waveform having a fundamental frequency component represented as fo (fp1/P) - fI, (fp1/P) > or = fI where fo equals the fundamental frequency of the AC output voltage waveform, fp1 is inversely proportional to the conduction interval T1 and fI is the frequency of the AC input power source, second conduction control circuit means for generating a second conduction control signal to control the ON-OFF conduction controlled bilateral switching means associated with said second converter circuit, each of said bilateral switching means of said second converter circuit being rendered conductive by said conduction control signals for a period of time, T2, to develop a second output voltage waveform having the same fundamental frequency component as said first output voltage waveform and represented as fo fI - (fp2/P), (fp2P) > or = fI where fo equals the fundamental frequency of the AC output voltage waveform, fp2 is inversely proportional to the conduction interval T2 and fI is the frequency of the AC input power source, means for combining said first and second output voltage waveforms having essentially the same fundamental component to form said AC output voltage waveform across said load, said first conduction control signal comprised of pulses at a frequency corresponding to the value of fp1 and said second conduction control signal comprised of pulses at a frequency corresponding to the value of fp2, said first conduction control signal resulting in the reflection of the negative of the load phase angle back to the AC input power source, said second conduction control signal resulting in the reflection of the load phase angle unchanged back to the AC input power source, the combination of said output waveforms forming said AC output voltage resulting in a net zero input phase angle for said converter circuit means and consequently a unity input displacement factor.

5. An AC to AC frequency changer apparatus as claimed in claim 1 further including a clock pulse generator circuit means operatively connected to said first and second conduction controlleD circuit for supplying a first pulse train to control said first conduction control circuit and a second pulse train to control said second conduction control circuit, said clock post generator circuit means comprising, means for producing AC timing waves which are opposite in phase to the voltage waveforms of said AC input power source, a reference generator means for generating an AC reference waveform having a frequency substantially identical to the desired output frequency of said frequency changer apparatus, a first sine wave crossing detector circuit for generating first output pulses corresponding to the intersection of said AC reference waveform with the negative slopes of said timing waveforms, a second sine wave crossing detector circuit for producing second output pulses corresponding to the intersection of said AC reference waveform with the positive slopes of said timing waveforms, gating signal means operatively connected to said sine wave reference generator means and producing a first output which is phase shifted by +90* with respect to said sine wave reference waveform and a second output which is phase shifted by -90* with respect to said sine wave reference waveform, and logic means responsive to said output pulses of said first sine wave crossing detector circuit means, said output pulses of said second sine wave crossing detector circuit means, and said first and second output signals of said gating circuit means to produce a first pulse train (P1) for controlling the operation of said first conduction control circuit means in accordance with the following relationship: P1 S1 . X + S2 . X and a second pulse train (P2) for controlling the operation of said second conduction control circuit means in accordance with the following relationship:P2 S2 . X + S1 . X wherein; S1 represents the output pulses of said first sine wave crossing detector circuit means and S2 represents the output pulses of said second sine wave crossing detector circuit means, and x represents the output signal of said gating circuit means which is phase shifted by +90* with respect to said AC reference waveform, and X represents the output signal of said gating circuit means which is phase shifted by -90* with respect to said AC reference waveform.

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